What are some good ways to find out, which Google keyword got searched the most in a specific area/city for a specific niche?

When it is about getting intended search volume or competition feedback with Google Planner (GP), the best way is to go deeper into Google's keywords database to fetch specific keywords for specific needs. Good enough, Google provides the tools with GP for doing that. However, many GP users don't use the tools. In answer to your question, take the following steps to refine your search so as to get the GP feedback on a specific area or city. a) Open the GP and enter your seed keyword. b) On the left sidebar, go to Targeting and click All Locations (See the image below) c) Input your intended area or city and choose the relevant area or city from the drop-down list. (See the image below).  Below I input New York. The highlighted item is the most relevant in terms of city. To accurately refine your search to get the most-searched keyword relating to your seed keyword, you need to  use additional GP tools (Keyword filters) to choose: a) the minimum search volume. b) the competition type. See below.   I input 1000 as the the minimum search volume. If this criteria is not set, usually a long list of GP keywords will be returned by the GP. Since what you want is the most-search keyword, I set the competition type to 'high' only. You may ignore the other two filters until you are deciding on bidding for the keyword you ultimately prefer. In your question, there are three 'how-tos' you are seeking: How to: a) focus your search on specific city. b) ascertain the most searched keyword. c) doing (a) and (b) within specific niche. The first two have been explained above and to do them within specific niche, just ensure that your seed keyword to be fed into the GP is highly relevant to your chosen niche. With my best wishes.

You can use https://adwords.google.com/KeywordPlanner, and http://www.google.com/trends/ for your specific keywords and specific city.

Use the Google keyword planner. Alternatively,  you can longtail your keyword/s. For example: If you were to search for "dresser store" in Google, it would come up with different results than if you typed "dresser store New York." You can also add identifiers, such as "unpainted dresser store New York." When you are doing research for specific keywords like this in the Google keyword tool, it often helps to click exact matches. If you are only trying to sell to locals, make sure you're not viewing global results. Tools like SEOquake will help you determine how hard it will be to compete for specific keywords. Many people will also make YouTube videos to rank for area specific keywords. If you own an autoshop, make a funny video using your keywords,  and spread it across all of your accounts (Twitter, Facebook, Reddit etc.). Get other people to share it. Embed it on your website.  Go crazy! I hope this helps.

The AdWords program provides this kind of granular information, if you sign up, and are interested in web commerce of some sort...  I'm not certain it gets THAT granular, (specific city/area/niche) but it gets pretty detailed.